1 It is larger than the other (roughly) 1000 KBOs discovered since 1992. Sedna is probably about 2000 km in diameter (Pluto is 2300 km). Scientifically, Sedna is most interesting because it might be large enough to retain a tenuous atmosphere, as does Pluto, at least at certain parts of its orbit when the temperatures are high enough for the gases to not be frozen.

2 Its distance from the Sun at discovery was greater than for any other KBO. This is not to say that other KBOs don't travel to larger distances than Sedna: they do. KBO 2000 OO67 travels out to 1000 AU (33 times the distance between the sun and Neptune), for example. It's just that these more extreme KBOs were discovered when closer to the Sun.

Is Sedna a Surprise?
No, it's an awesome thing to find, but it is not a surprise. This is because planetary astronomers have worked hard to measure the size distribution of KBOs and have expected for years to find objects of Pluto's size and perhaps a little larger. Sedna fits in with the measured size distributions.

Is Sedna a Planet?
Like Pluto, Sedna is most usefully regarded as a big Kuiper Belt Object. But if you think Pluto is a planet, you should probably label Sedna that way too, because it's almost as big. Get ready for a 20-Planet Solar System!"

There should be no surprise that there are other planets. Gravitation has no cutoff point, AFAWK. The only gravitational rival would be other stellar masses, and they are a light-year or more away. Traditional planets were known because they could be easily spotted. Even Uranus can be seen by sharp-eyed viewers who know where to look under excellent seeing conditions. Neptune is just a bit too far away for that and a telescope is needed. Pluto is too far away and too small, so you really need to know where to look. Other planet-like bodies farther out need luck to find them, but astronomers are always looking. Also, astronomers now have more resources for detecting presence of compact masses than previous astronomers had. The profound cold at significant trans-neptunian distances makes accretion into planetoid bodies likely. But proving they are planetary (around our sun) becomes more difficult with distance).

Here's the http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/release.shtml [Broken] about Sedna.

The discoverers consider it to be evidence of the inner part of the Oort Cloud rather than Sedna being a Scattered Disk Object.

How does this relate to the Hubble findings last year, namely that the (classical) Edgeworth-Kuiper Belt seems to have an outer edge, at ~50 au? For example, might there be thousands of Sedna-like objects, which give rise to the outer edge of the EKB?

When Clyde Tombaugh discovered Pluto in around 1928, he classified Pluto as a planet, especially considering that he first thought that Pluto was the size of Earth (e had no idea of the Kuiper Belt back then either). But now we know better; and I do believe that Pluto really was just named a planet just because of incomplete facts. Pluto acts more like a Kuiper Belt object than a planet.

Originally posted by Nereid Here's the http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/release.shtml [Broken] about Sedna.

The discoverers consider it to be evidence of the inner part of the Oort Cloud rather than Sedna being a Scattered Disk Object.

How does this relate to the Hubble findings last year, namely that the (classical) Edgeworth-Kuiper Belt seems to have an outer edge, at ~50 au? For example, might there be thousands of Sedna-like objects, which give rise to the outer edge of the EKB?

I *like it* when some of my musings turn out to have people like Jewitt say similar things . From the same Jewitt site as earlier, updated:

"Why is Sedna Interesting?
1 Its perihelion (closest approach to the Sun) is at 76 AU. This means that it is effectively beyond the scattering influence of Neptune. This is unlike the Classical KBOs, and unlike the Scattered KBOs. It is similar, dynamically, to 2000 CR105 (for which a/e/i = 227AU/0.805/22.7) which has perihelion at 44 AU, also outside Neptune's reach, and which has been discussed in papers by Gladman et al (Icarus 157, 269, 2002) and Emel'Yanenko et al (Monthly Notices RAS, 338, 443, 2003). Other objects have larger aphelia than Sedna's 990 AU (e.g. Kuiper Belt Object 2000 OO67, with aphelion at 1010 AU) and many comets travel to larger distances. Sedna is interesting because of its perihelion distance.

2 Sedna is large (1000 - 1500 km). An object this large cannot have formed by accretion in the tenuous regions of the protoplanetary disk corresponding to its current location. Sedna must have formed elsewhere, presumably amongst the planets or in the Kuiper Belt, and been ejected outwards. Lastly, its perihelion was lifted out of the range of Neptune. The orbit and the size attest to an early epoch in which strong gravitational scattering events rearranged the small bodies of the solar system.

Is Sedna an Oort Cloud Comet?
From the Classical Oort Cloud - no. The latter consists of objects whose orbits are so large (50,000 AU) that passing stars and galactic tides can alter their properties. Sedna doesn't travel very far out (1000 AU) and is effectively immune to external forces. Also, the inclinations of both Sedna and 2000 CR105 are small (12 and 23 degrees, respectively). These objects know where the plane of the solar system lies. Oort Cloud orbits are random with inclinations all the way up to 180 degrees.

So What Is It?
Sedna could be a member of a substantial population of bodies trapped between the Kuiper Belt and Oort Cloud. These would have been emplaced at early times and unseen until recently. 2000 CR105 and Sedna are "just the tip of the iceberg", as they say. The scientific interest lies in how these objects had their perihelia lifted out of the planetary region."